To backup a horde environment, these items need to be considered:

Config files

As of Horde 6, it is sufficient to backup the var/config/ folder.
You may also want to safe the SSL certificate files and the vhost config of your web server.
You should also safe your composer.json and composer.lock files.

Database backup

Backup your SQL and NoSQL databases using the applicable technologies.
In case of mariadb and mysql it is also useful to safe the database server's config files.

IMAP server

Backup your accounts on your imap server. This is usually outside the Horde administrator's responsibility. However, a webmail client like horde/imp is perfectly capable of trashing your logged in user's mailbox.

Local filesystem

If you use the virtual filesystem, i.e. for email attachments, ansel photo gallery or gollem file manager, you need to backup the VFS tree.
The recommended path for VFS in Horde 6 is the var/vfs/ subdir of your root project but admins may configure different locations. Consult the config file in var/config/horde/conf.php

$conf[vfs][params][vfsroot] = 

LDAP server

If you use LDAP for authentication or for addressbook storage, ensure a regular backup exists. This is usually outside the horde administrator's responsibility.